docs: def assert_is(first, second, msg_fmt="{msg}"):
      """Fail if first and second do not refer to the same object.
  
      >>> list1 = [5, "foo"]
      >>> list2 = [5, "foo"]
      >>> assert_is(list1, list1)
      >>> assert_is(list1, list2)
      Traceback (most recent call last):
          ...
      AssertionError: [5, 'foo'] is not [5, 'foo']
  
      The following msg_fmt arguments are supported:
      * msg - the default error message
      * first - the first argument
      * second - the second argument
      """
  
      if first is not second:
          msg = "{!r} is not {!r}".format(first, second)
          fail(msg_fmt.format(msg=msg, first=first, second=second))